Non-destructive measurement of a film thickness is an important objective for many industries. For example, in the aircraft industry paint for functional and decorative purposes is often applied to control surfaces which may have near critical weight. Also, prior to applying a surface paint, care must be taken to make sure that an adequate amount of primer is applied. To act as a proper corrosion inhibitor the primer must typically be applied uniformly to a desired thickness. For example, in some aircraft applications, a desired thickness is between 0.4 and 1.0 mils (thousandths of an inch) to the surface of the substrate. Moreover, depending on the model of aircraft over 500 pounds of primer paint may be applied to each aircraft according to customer specifications. Since the weight and distribution of paint on control surfaces may sometimes affect the performance of the aircraft it is desirable to apply and measure the paint in a controlled means and generally to keep the thickness to a minimum. This measurement is desirable to insure that the primer is sufficiently thick for corrosion protection, but not so thick as to unnecessarily add to the weight of the airplane and affect fuel efficiency. After the primer thickness is determined to be of sufficient thickness, a top coat can be applied.
Control surface components that are made of a metallic material can have their coatings measured by several conventional commercial tools. However, increasingly more components are now manufactured from composite materials such as carbon fiber epoxy or fiberglass. These materials are difficult to measure using conventional techniques since they are only semi-conductive at best.
Another conventional alternative is to wait for the paint to dry and measure thickness indirectly by weighing the component in question. This can result in an undesirable cycle delay of, for example, up to three hours. When primer needs to be measured on a composite component, the component is first weighed and then painted. Since no direct measurement technology is available for composite materials, when the primer is suspected of being too thick or uneven, the component typically must be disassembled and repainted at significant cost and risk of damage.
Commercial ultrasonic measurement systems, such as Panametric 45L Deluxe®, are available which give some paint measurement thickness data for some types of composites. The Panametric 45L Deluxe® is useable for relatively thick coatings, but for thin coatings of paint this ultrasonic system is not sufficiently reliable to meet usual aircraft specification requirements.
Additionally, the non-destructive measurement of film thickness is of concern to other industries such as both the automotive industry and the paper production industry. In the automotive industry, the analysis of the primer paint thickness is important, for example, to determine whether additional touchup work is required for some of the exotic paint applications common to the industry. In the paper industry, maintaining a check on the uniformity of the paper with a sensor can be an important quality control issue. In both these industries, and with the increasing use of composite materials, it has become apparent that current methods of measurement that work on metallics or thick films but not on semi-conductive materials or thin films are not sufficient.
Therefore, there is a need in various industries for systems and methods that are capable of non-destructively measuring the thickness of thin film materials and verify that the thickness is within specifications.